It is known that some aspects of graphics processing are computationally intensive tasks. For example, in a graphics processing system which receives graphics primitives, performs rasterization operations on those primitives to generate graphics fragments, and then performs rendering (shading) operations on those graphics fragments, the latter rendering (shading) operations are known to be particularly computationally intensive. For this reason, it is further known to seek to identify as early as possible in a graphics pipeline any graphics fragments which will not appear in the final display because they are overdrawn by other graphics fragments. If such hidden graphics fragments can be identified early in the graphics processing, in particular before the shading operations are carried out, then significant unnecessary processing can be avoided.
One aspect of identifying such hidden graphics fragments and avoiding further processing on them is known as forward pixel killing. This typically comprises the graphics processing system having depth testing capability which enables it to determine, for multiple graphics fragments corresponding to a particular display location, which of those graphics fragments will be front-most (and therefore should be fully processed and displayed) and the remaining graphics fragments which will be hidden (and therefore further graphics processing with respect to these hidden graphics fragments can be avoided). It is thus known to generate a “forward pixel kill” signal, when a graphics fragment is identified which is known to be front-most, which will cause the graphics processing system to abandon further processing with respect to any graphics fragments ahead of that front-most graphics fragment in the graphics pipeline.
An alternative approach to the problem of not performing graphics processing with regard to components which will ultimately be hidden is for the graphics primitives received by the graphics processing system to be fully sorted in terms of viewing depth (z sorting) such that the graphics processing can be performed in a strictly front-to-back order, and the graphics processing can simply be halted when all of the front-most components have been processed.